This invention relates to a method of making a product and more particularly but not exclusively to a method of making a building product such as a tile.
In our previous patent GB 2244051 there is disclosed a method of making a building product from slate particles which in the end product are bonded together by a fused glassy binder. In the examples described, the binder comprises recycled glass or sodium silicate for examples. When such products are fired, the recycled glass melts or the silicate of the sodium silicate fuses wuth the result in each case that the slate particles are held in a matrix of fused glassy binder.
It was found that in such a method, the use of sodium silicate as a binder rather than recycled glass was preferred as the end (building) product was of a more consistent quality. However sodium silicate is an expensive raw material.
According to the first aspect of the present invention we provide a method of making a product comprising the steps of making a generally homogenous mixture of slate particles and clay powder, wetting the mixture as necessary to produce a formable composition, forming the wet mixture to a desired configuration, at least partially drying the formed mixture and subjecting the dried formed mixture to a temperature of at least 1100xc2x0 C. for a time sufficient for at least some of ther clay to convert into a binder to bind together the slate paticles.
Utilising the method of the invention, a product of consistent quality may be made without requiring expensive binder raw material, but rather, relatively inexpensive clay may be used. Also it has been found that a greater proportion of slate may be used than in the prior method which results in an improved quality product, and it is possible to produce a thinner product with a mechanical strength equal to thay of thicker products produced by the prior art method.
It will be appreciated that clay comprises typicall ablout 48% silicates and that the firing process e.g. in a kiln will cause these silicates to be converted to a binder by fusing.
The slate particles may comprise fragments, powder and dust all of which are able to pass through a sieve of 6 mm preferably 3 mm size and more preferably 2 mm size and if desired slate particles at least some of which are too large to pass through such a size sieve may be subjected to a mechanical treatment to reduce the particles to slate particles all of which are able to pass through the sieve of 6 mm, preferably 3 mm and r.ore preferably 2 mm size. For example the particles may be fed between sizing rollers which break larger fragments.
The slate particles may comprise a mixture of particles including mill fines, e.g. at least 40% mill fines by weight. At least 10% and more preferably at least 20% by weight of the slare particles may comprise dust/powder having a maximum dimension in the range 0.075 mm to 1 mm.
The homogenuos mixture prior to coming is preferably generally dry but particularly where the slate particle include dust which presents a potential health hazard, the slate particles may include some water although the mixture will in general not be formable until wetted.
Conveniently to facilitate mixing a generally homogenous mixture, the clay to be mixed with the slate particles comprises powder which is all able to pass through a sieve of size 6 mm, preferably 3 mm and more preferably 1 mm size.
In one embodiment the solid constituents of the mixture may contain only slate particles and the solid constiuents of the mixture may comprise between 0.5% and 15% by weight, preferably between 6% and 15% by weight cloay powder, and between 85% and 95% by weight slate particles. In one example the solid constituents of the mixture comprised 13% by weight, clay powder. To aid binding if desired some talc may be added to the mixture such that the solid constituents of the mixture comprise at least 1by weight, talc.
The formable wet mixture may comprise between 0.5% and 19% by weight water, the amount of water used depending on the proportion of larger particles in the mix and the volume of the mix, so as to form a wet mixture which may conveniently be formed to the desired configuration and will retain its formed configuration well.
Preferably the wet mixture is formed to the desired configuration by moulding under pressure.
In one embodiment the wet mixture is moulded by forming the mixture to a generally ball shape, placing the ball shape in a mould and applying pressure to the ball shape such that the wet mixture fills the mould. In another embodiment the wet mixture is formed to the desired configuration by extruding the wet mixture through a die to form an, e.g. generally rectangular cross section, extruate, cutting the extruate to a desired length and placing the cut length of the extruate in a mould and subjectting the extruate to pressure such that the wet mixture fills the mould.
In each case, the formed wet mixture may be removed from the kiln the wet mixture becomes at least partially dried. This movement towards the kiln the wet subject the wet formed mixture to a temperature gradient, the temperature increasing as the formed wet mixture moves towards the kiln.
This movement towards the kiln may be very slow but by the time the formed mixture enters the kiln, the water content may be reduced to below 10% by weight, for example to substantialy 0% if desired.
If desired, the dried formed mixture may be sujected to a temperature in excess of 1000xc2x0 C. e.g. at least 1145xc2x0 C. for a time sufficient for at least some of the slate particls to be converted into a binder to bind together the remainder of the slate particles.
The product may be of any desired configuration for its intended use but the invention is particularly applicable where the product is of generally flat configuration.
Thus the product may be a building element such as a tile for roofing, flooring, and cladding for examples only.
According to a second aspect of the invention we provide a method of making a product comprising the steps of making a generally homogenous mixture of slate particles, wetting the mixture as necessary to produce a formable composition, forming the mixture to desired configuration, at least partially drying the formed mixture and subjecting the dry formed mixture to a temperature of at leat 1145xc2x0 C. for a time sufficient for at least some of the slate particles to be converted into a binder to bind together the remainder of the slate particles.
In such a method, there is thus no need to use any binder in addition to the slate particles.
The mixture may included feldspar e.g. of at least 15% by weight, which fuses as the dried formed mixture is heated, into a binder.
According to a third aspect of the invention we provide a method of making a product comprising the steps of mixing together slate particles and feldspar particles, heating the mixture sufficiently for the feldspar to melt and fuse, permitting the material to cool, to bind the slate particles together.
The second and third aspects of the invention may utilise any of the features of the method of the first aspect of the invention.